The present invention relates to a control circuit for a least one semiconductor switch in which uncontrolled switching of the semiconductor switch due to the presence of noise is inhibited.
There are many applications in which it is desirable to switch a voltage or load using a control signal. A common use of such a switching arrangement is to switch a supply voltage on and off to an external or internal load. An occurrence of this application is within the aerospace industry. Previously it was well known to accomplish such switching using an electromechanical relay. However, the electromechanical relay has been replaced in many applications by semiconductor switches to achieve the higher reliability, lower cost, and ease of manufacture that semiconductor switches offer.
The use of semiconductor switches as replacements for a mechanical relay can however also pose problems when a semiconductor switch not directly connected to a common ground or supply rail is required. When using such a floating xe2x80x9chigh sidexe2x80x9d semiconductor switch it is desirable to limit as far as possible spurious or uncontrolled switching of the semiconductor switch due to the presence of noise. Although such noise isolation may be achieved using wound or optically isolated semiconductor switches, such devices are ordinarily prohibitively expensive and complex for most applications. A known approach has therefore been developed that makes it possible to drive a floating xe2x80x9chigh sidexe2x80x9d power semiconductor switch whilst avoiding transformers, opto-isolators or individual DC to DC derived supplies. This known approach involves deriving a control voltage that is applied to the semiconductor switch using a charge pump. As it is necessary that the switching circuit does not rely on the output load or supply for its operation, it is necessary to use a differential drive method to derive the control voltage. Using this arrangement, it is known to provide a capacitive separation barrier to provide some noise isolation and DC blocking between the differential drive circuit and the overall power supply. The use of such a capacitive separation barrier allows the semiconductor switch and its control circuitry to float with respect to DC or low frequency AC voltage, whilst the power and control signals are transferred in a higher frequency domain.
The problem with capacitive separation alone is if large amplitude AC signals are present on the supply line or load they can cause spurious turn on of the semiconductor switch, or more problematically, can prevent the semiconductor switch from turning off. This is a problem on DC systems but is even more acute with high voltage AC supplies, such as 115V 400 Hz AC supplies commonly found in aerospace applications.
Further problems associated with floating semiconductor switches are the inductively induced switching transients that can cause breakdown of the semiconductor switch. On DC systems it is possible to deal with load transients by means of freewheeling diodes, but this is generally not an optimal solution. Furthermore, freewheeling diodes do not control line-based transients produced when the switched supply either has a long run of cabling or is derived from a transformer rectifier unit (TRU). On AC systems, freewheeling diodes are not applicable and either active or passive snubbing is required to avoid uncontrolled avalanche conditions in the power switch elements.
The alternative is to accommodate such line and load induced transients using the semiconductor switch itself to actively limit the transient voltages and capacitive input current surges or absorb inductive energy. Such a protection scheme has the advantage of high speed inductive load switching, thus providing a closer approximation of relay type operation. However, the problem with this form of self-protection is that it may not provide adequate protection against lightning strikes. The ability of a semiconductor switch to absorb the energy contained in a lightning strike is limited by, first the physical size of the semiconductor switch and second the required clamping voltage. These limitations means that, in certain safety critical applications, the semiconductor switches would have to be vastly overrated to provide the necessary self-protection, leading to increased costs and physical size of the circuits.
According to the present invention there is provided a control circuit for at least one first semiconductor device, the circuit comprising: a rectifier circuit having first and second inputs and arranged to generate a control signal for the at least one first semiconductor device when the first and second inputs are driven in anti-phase; and a validation circuit coupled to said rectifier circuit and the or each first semiconductor device, said validation circuit being arranged to inhibit uncontrolled switching of the or each first semiconductor device in the presence of noise.
The provision of a validation circuit interposed between the rectifier circuit and the semiconductor switch inhibits uncontrolled switching of the or each semiconductor device, which preferably acts as a switch. A further advantage is that the use of additional devices is avoided to provide protection against lightning strikes.
Preferably the validation circuit comprises a second semiconductor device arranged to allow charge flow to a charge store only when an alternating voltage difference between the first and second inputs of the rectifier circuit exceeds a first threshold. It is therefore possible to provide a control circuit that exhibits good common mode noise rejection because only differential voltages are permitted to be conducted by the validation circuit.
Preferably, the charge store is connected to a control terminal of the or each first semiconductor switch. The charge store provides a time delay in switching off or on the or each first semiconductor switch that tends to counteract any uncontrolled operation of the semiconductor switch.
Preferably, the or each first semiconductor switch is a field effect transistor. FET""s provide a cost effective solution whilst exhibiting good switching characteristics, and low xe2x80x9cONxe2x80x9d resistance.
Preferably, the first and second inputs of the rectifier circuit are high pass filtered. This further improves the noise rejection capabilities of the circuit.
Preferably, the validation circuit is arranged such that a negative voltage transient, for example due to a lightning strike, applied to the or each first semiconductor switch biases said second semiconductor device off. The validation circuit is therefore arranged to actively bias the second semiconductor device off in the event of a negative voltage strike, thus preventing uncontrolled turn on of the field effect transistor. Additionally, the validation circuit may further comprise a third semiconductor device arranged such that a positive voltage transient, for example due to a positive lightning strike, applied to the or each first semiconductor device biases the third semiconductor device off. The validation circuit therefore provides controlled protection of the field effect transistor in the event of both negative and positive transients which may arise as a result of lightning strikes.
Preferably, the control circuit further includes an inductive load protection circuit for modifying the rate of turn off of the or each first semiconductor switch so as to limit the voltage appearing across the switch when switching loads having a significant imaginary component as part of the load impedance.
Additionally, the protection circuit may comprise a unidirectional current flow path connection between a control terminal of the or each first semiconductor switch and a node formed between a capacitor and a resistor, a capacitor being in connection with a first terminal of the semiconductor switch and the resistor being in connection with a second terminal of the semiconductor switch.
It is therefore possible to additionally provide both absolute voltage clamping and rate of change of voltage clamping when switching inductive loads without the use of freewheeling diodes.